The present invention relates to a patterning process in the fabrication of VLSI, LSI and IC systems and more particularly a process capable of transferring the pattern with extremely fine features to the wafer surface.
The recent trend of semiconductor devices is toward increasingly higher densities. With conventional photolithography with ultraviolet light (of the wavelength of 300-400 nm), the highest obtainable resolution is limited to one micrometer and consequently the feature size is limited to 2-3 micrometers. In order to attain a resolution higher than can be achieved by photolithography, extensive research and development has been carried out in order to perfect the X-ray and electron beam lithography techniques, which use shorter wavelengths than photolithography.
Electron beam lithography uses a beam of charged electrons, so that it has an advantage of being capable of easily controlling the electron beam electrically. Therefore, a digital process can be used under the control of a computer to draw a pattern on a wafer. However, the emitted electrons are electrically charged so that when they strike a resist layer, they interact with molecules in the resist layer and consequently are scattered until they lose their energy and finally are trapped somewhere. Because of this scattering effect together with the backscattering effect, the highest obtainable resolution is 0.1 micrometer.
In the case of X-ray lithography, X-rays are not electrically charged, so that the pattern features are distorted only by the secondary electrons emitted and scattered as a result of bombardment of nuclei in a resist material by the X-rays. X-ray lithography can, therefore, attain a high resolution of 0.01 micrometer. However, as described above, the X-rays are not electrically charged so that they cannot be electrically controlled and consequently the scanning exposure process used in electron beam lithography cannot be used for X-rays. It follows, therefore, that X-ray lithography requires specially designed masks. Masks for the X-ray lithography are prepared by forming a pattern in the form of an extremely thin film of gold or the like over the surface of a substrate of silicon or other high-molecular weight material.
As shown in FIG. 1, the thus prepared mask 1 is placed in an X-ray lithography device or machine comprising an exposure chamber 24 and an X-ray source chamber 23 with a target 21 and an electron gun 22. A wafer 2 which is supported on a wafer holder is spaced apart from the mask 1 by S (which is typically from 5 to 25 micrometers). The wafer 2 is exposed through the mask 1 with soft X-rays (0.2-1.5 nanometers in wavelength) emitted from the target 21. However, factors such as the diameter d of the target 21 or the initial diameter of the emitted beam of X-rays, the radiation angle .theta. and the spacing S between the wafer 2 and the mask 1 combine to give rise to undesired exposures; that is, distortions or blur .delta. to the edge profile of the beam.